1. Field of the Inventions
The inventions relate to methods of optimizing the geometry of femoral stems of hip joint prostheses for implantation into a femur, femoral stems of hip joint prostheses produced by said methods and to hip joint prosthesis systems for use in the implantation of said femoral stems into a femur.
2. Description of the Related Art
A femoral stem of a hip joint prosthesis for implantation in a femur comprises a neck and an anchoring portion in the form of an anchoring blade that tapers towards a distal end. Such stems are known, for example as described in U.S. Pat. No. 4,908,035, which is incorporated by reference herein in its entirety. In order to implant such a stem during surgery, the hip joint is opened and the neck of the femur is resectioned. The proximal femur is then prepared for receiving the anchoring blade. This preparation involves the creation of a bony anchoring bed within the proximal femur by using suitable shaping instruments, in particular one or more appropriately shaped rasps. Each rasp is usually hammered down into the medullary space of the femur that is filled with spongy bone and soft tissue in order to scrape or rub away the bone so that ultimately an anchoring bed is produced that conforms in shape to the shape of the anchoring blade of the prosthesis stem. If the stem is to be implanted by cementless anchoring, then the blade of the stem is preferably straight, as described in U.S. Pat. No. 4,908,035. In this case, as shown in FIG. 1, the anchoring bed of the femur 1 requires the medullary space 2 to be opened not only in the plane 3 of the resection surface of the neck of the femur 1, but also further laterally into the region of the greater trochanter 4 to provide an axial anchoring bed for the anchoring blade.
While such implantation techniques produce a stable joint with good bone ingrowth behaviour after implantation, they also involve resections and detachments of tendon and muscle insertions in the region of the greater trochanter 4. This necessarily involves significant operative trauma, particularly to functionally significant structures such as the tendons and muscles as good operation of the hip joint after such surgery relies on the functionability of these muscles and tendons.
More recently, there has been an increase in hip prostheses using minimally invasive surgical techniques. The aim of such techniques is a more a rapid rehabilitation of the patient, which is associated with a reduction in pain and a shorter stay in hospital. Such minimally invasive surgical techniques try to avoid resections of tendons and muscles in the region of the greater trochanter 4. This usually means that instead of producing an anchoring bed as shown in FIG. 1, a bed is produced similar to that shown in FIG. 2, wherein it can be seen that the greater trochanter 4 is left substantially undisturbed, the medullary space 2 being opened in the plane 3 of the resection surface and the greater trochanter 4 being undercut, as at an undercut profile 5 in a proximal lateral region of the space. This creates an anchoring bed into which a femoral stem of a hip joint prosthesis stem must be implanted that is substantially axial over the greater part of its length but that has a slightly curved or angled proximal portion. In such a stem some or all of a trochanter wing of a blade of the stem has to be omitted. An embodiment of such a femoral stem 6 of a hip joint prosthesis is shown in FIG. 3 and is described in US Pat. App. Pub. No. 2009/0036994, which is incorporated by reference herein in its entirety. The aim of this shape of prosthesis is to provide an anchoring blade suitable for implantation by minimally invasive surgical techniques while retaining as far as possible the advantages of conventional straight stem implants.